This invention relates to a rotary apparatus such as hard disc drive (HDD), a spindle motor used as a drive source for such a rotary apparatus and to a fluid dynamic pressure bearing adapted for such a spindle motor bearing. More particularly, the invention relates to a shaft-both-end fixed type fluid dynamic pressure bearing to be fixed at respective ends of its shaft onto a chassis, etc. of a utilized apparatus through screws or the like.
Air dynamic pressure bearings are broadly used in rotary apparatuses such as a HDD, a optical disc drive and light polarizing apparatuses because of such as their light weight of light, clean and smooth rotation, durability to heat and cold, long service life and freedom of contamination to storage media such as discs, by virtue of the nonuse of lubrication oil. Recently, however, there has been a significant increase in the information amount required to be processed. Particularly, the large capacity HDD apparatus is required to rotationally drive as many as five or more discs. This requirement an no longer be met by an air dynamic pressure bearing. To cope with this, fluid dynamic pressure bearings have been adopted for HDD apparatuses that can support load weight greater than that of the air dynamic pressure bearings.
There are disclosures of basic structures and operations of fluid dynamic pressure bearings, e.g., in U.S. Pat. No. 5,112,142; U.S. Pat. No. 5,524,985; U.S. Pat. No. 5,524,986; and U.S. Pat. No. 5,533,812.
The conventional fluid dynamic pressure bearings, particularly the fluid dynamic pressure bearings to rotate the sleeve, includes two kinds of devices, if classified by the manner of fixing the shaft onto a utilized apparatus. One is a shaft-one-end fixed type fluid dynamic pressure bearing as shown in FIG. 18, and the other is a shaft-both-end fixed type fluid dynamic pressure bearing as shown in FIG. 19. First, the fluid dynamic pressure bearing of FIG. 18 is structured by a fixed shaft 1 to be fixed at its lower end onto a chassis 16 or the like through a screw 15, and a rotary sleeve 2 having an upper end covered completely by a lid member 20 and a lower end having an opening 11 forming a capillary seal. Next, the fluid dynamic pressure bearing of FIG. 19 is structured by a fixed shaft 1 to be fixed at respective ends onto a chassis 16 or the like of a utilized apparatus through screws 14 and 15, and a rotary sleeve 2 having openings 11a and 11b forming respective capillary seals at upper and lower ends.
In FIG. 18 and FIGS. 19, 8, 8a, and 8b are radial dynamic pressure producing grooves, while 9a and 9b are thrust dynamic pressure producing grooves. 5, 5a, 5b, 17a, 17b and 17c are fine gaps formed between the fixed shaft 1 and the rotary sleeve 2. These fine gaps are filled therein with lubrication oil 18. The fine gaps have a width of usually 2 to 15 .mu.m, although depending on the size of the fluid dynamic pressure bearing. 13a is an upper screw hole of the fixed shaft, while 13, 13b denote a lower screw hole.
In the shaft-one-end fixed type fluid dynamic pressure bearing of FIG. 18, the lubrication oil 18 filled within the fine gaps 5, 17a, 17b and 17c is in contact with the air at the tapered opening 11. However, the filled lubrication oil 18 is prevented from leaking to an outside of the fine gap by the presence of a capillary seal and surface tension in this opening. In particular, the fine gaps 17a, 17b and 17c constitute a closed end.
The filled lubrication oil 18 is made difficult to leak out through the opening 11 due to a fine gap structure having the closed end, i.e. a fine gap structure with one-side closure. In the shaft-both-end fixed type fluid dynamic pressure bearing of FIG. 19, on the other hand, the filled lubrication oil 18 filled within the fine gaps 5a, 5b, 17a, 17b and 17c is in contact with the air at the tapered upper opening 11a and lower opening 11b. However, the filled lubrication oil 18 is prevented from leaking out of the fine gaps by the presence of a capillary seal and surface tension given by the openings.
Of the above related-art apparatus, the shaft-one-end fixed type fluid dynamic pressure bearing of FIG. 18 has a closed end formed in the fine gaps. Accordingly, even if the apparatus is tilted, the lubrication oil does not easily leak out. Thus, the apparatus is excellent in sealability. However, there is a disadvantage in that the shaft 1 is fixed at only one lower end point and hence may undergo precession motion during rotation at high speed, resulting in unstable rotation. Conversely, the shaft-both-end fixed type dynamic pressure bearing of FIG. 19 fixes the shaft 1 at its both ends and hence will not undergo precession motion during rotation at high speed, thus offering stable rotation. However, there is a problem in that the fine gaps are opened to the air at upper and lower sides, resulting in insufficient of sealability. Even if the surface tension is increased by providing an air reservoir within a fine gap between the upper and lower radial dynamic pressure producing grooves 8a and 8b, the surface tension is abruptly decreased upon tilting and horizontally positioning the fluid dynamic pressure bearing. Furthermore, if in this state temperature change or external impact is applied, the lubrication oil filled within the fine gap may readily leak out.